cmd/5l: fix arm build

Fixes build. One part of ../misc/cgo/test appears to be unhappy but this does not cause the build to fail.

R=rsc, r
CC=golang-dev
https://golang.org/cl/7769047

src/cmd/5l/obj.c

@@ -136,7 +136,7 @@ main(int argc, char *argv[])
 	if(argc != 1)
 		usage();
 
-	if(linkmode != Linkinternal) {
+	if(linkmode != LinkInternal) {
 		diag("only -linkmode=internal is supported");
 		errorexit();
 	}